Recently, an organic electroluminescent (EL) element has been studied which is used in a display such as a flat display, or in a light source for an electrophotographic copier or a printer.
An organic EL element has a structure in which a layer containing an organic fluorescent compound is arranged between a cathode and an anode, and an electron and a positive hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by deactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several to several decade volts. The element has a wide viewing angle and a high visuality since the element is of self light emission type. Further, the element is a compact, complete solid element, and is noted from the viewpoint of space saving and portability. Therefore, study for practical application of the element has been made.
Recently, a phosphorescence organic EL element employing an excited triplet state, which surpasses a conventional organic EL element employing an excited singlet state, has been proposed by S. R. Forrest et al. (Appl. Phys. Lett. (1999), 75 (11), 4-6). Further, an organic EL element attaining a visibility efficiency of 60 lm/W was reported by C. Adachi et al. (J. Appl. Phys., 90, 5048 (2001)). Application of the organic EL elements described above to an illuminator as well as a display is expected.
However, when an illuminator employing an organic EL element is manufactured, the following must be considered.
As organic electroluminescent materials (EL materials), there are lower molecular weight EL materials and high molecular weight EL materials. When the lower molecular weight EL materials are used in the manufacture of an EL element, vacuum deposition is carried out. The lower molecular weight EL materials, which can be purified by sublimation, are easy to be purified, which provides El materials with high purity. Further, the lower molecular weight EL materials are easy in obtaining a multilayer structure, and are excellent in productive efficiency or lifetime. However, since deposition is carried out under high vacuum condition of a pressure of not more than 10−4 Pa, operation is complex, and expensive. Accordingly, the deposition method is not necessarily preferable in the manufacture. Particularly, a large size EL element, which is necessary for illuminator use, is difficult to manufacture. Further, regarding phosphorescent dopants, which are used in a phosphorescence organic EL element, it is difficult to form plural layers with a large size of the phosphorescent dopants without unevenness by vacuum deposition, and obtain an organic EL element. The vacuum deposition is disadvantageous in cost of manufacture and in manufacturing technology.
In contrast, when the high molecular weight EL materials are used in the manufacture of an EL element, a wet process such as a spin coating method, an ink jet method, or a printing method can be used. The wet process can be carried out under atmospheric pressure, has an advantage in low cost. Further, it can form a thin layer employing a solution of the materials, it has advantages in that dopants can be easily controlled and a large sized layer without unevenness can be easily obtained. When an illuminator employing an organic EL element is manufactured, this process is advantageous in cost of manufacture and in manufacturing technology.
However, the wet process employing the high molecular weight EL materials is difficult in forming a multilayer structure. When a second coating solution of a second high molecular weight EL material is coated on a first solution of a first high molecular weight EL material, the first high molecular weight EL material is dissolved in a second solvent used in the second coating solution, the first and second materials are mixed. This lowers productive efficiency as compared with the process used in the manufacture of an EL element employing the lower molecular weight EL materials.
Generally, an organic EL element comprising high molecular weight materials is manufactured by a spin coating method, an ink jet method or a printing method.
The organic EL element cab be manufactured sheet by sheet by the spin coating method, but cannot continuously. The ink jet method is useful for the manufacture of a display emitting three color lights, but is disadvantageous in productivity to the manufacture of a luminator or a luminous device emitting the same color light such as a display of a color conversion type.
A printing method is disclosed in Japanese Patent O.P.I. Publication Nos. 3-269995, 10-77467, and 11-273859. The printing method is a simple and useful method. However, the printing method is a method forming a layer one by one, and when two or more layers are formed according to the printing method, one layer is formed, and then another layer is formed on the resulting layer. This not only lowers productivity but increases cost due to increase of manufacturing installations or processes. Further, when layers of an organic solvent soluble polymer are multi-layered, there is a problem in that two layers are mixed.